An earthquake detector which can detect the occurrence of noticeable earthquakes is useful to awake persons who will sleep through an earthquake shock, and to shut off gas and/or electricity to minimize the possibility of fires. An earthquake detector can be attached to a main structural component of a building or other structure so that it shakes or vibrates when the building vibrates, to generate a signal that sets off an alarm. However, it is important that the detector not give a false alarm in the event of other common non-earthquake vibrations such as slamming of a door, running in the building, nearby heavy traffic, or a sonic boom. An earthquake detector which could generate a signal upon the detection of a moderate earthquake, but which was resistant to the generation of false alarms, would be of considerable value.
Earthquakes produce P (for primus)-waves and S (for secundus)-waves. The P-waves travel at a rate of about 5.6 km per second (about 3.5 miles/sec.), while the S-waves travel at a velocity a little more than half that of the P-waves (about 1.8 miles/sec.). The P-waves are less energetic than the S-waves, but can serve as a warning to the imminent arrival of more powerful S-waves, if a detector was especially sensitive to a P-wave. A relatively simple apparatus which was especially sensitive to P-waves, would be of considerable use in warning persons of impending large shock waves.